The present invention relates generally to permanent magnet rotors, and more particularly to a novel permanent magnet rotor employing a deep drawn cup or shell and a method for encapsulating into the shell a core having magnetizable elements held about the circumference of the core.
Permanent magnet rotors for electronically commutated motors (ECMs) typically are constructed of a generally cylindrical iron core, which may be of a solid or sintered metal construction or may be a laminated core, and around which are positioned a plurality of magnetizable elements. The magnetizable elements are arcuate shaped with an inner contour conforming to the outer surface of the core such that the magnets may be adhered to the core, as by an adhesive layer. The magnetizable elements are generally made from barium or strontium ferrite, or samarium cobalt (rare earth). Permanent magnet rotors of this general type find particular application in motors having control circuitry which controls energization of the windings in one or more predetermined sequences to provide rotational magnetic fields and thereby rotation of the rotor. Numerous patents disclosing such rotor applications are identified in U.S. Pat. No. 5,040,286 which is assigned to the assignee of the present invention and incorporated herein by reference. Rotors produced in accordance with the present invention can be used to particular advantage in refrigeration evaporation and condenser fan motor applications where high reliability, high efficiency and long service life are particularly important.
The relative high mass of the materials used to form the magnetic elements and the relatively high rotor speeds, generally in the order of 1000-16,000 RPM, create significant forces during rotor operation, including centrifugal forces and substantial momentum forces upon sudden reversals of rotor rotation. Retention of the magnets in predetermined rotational positions about the core is critical to optimum motor operation. A number of methods and techniques for retaining magnets in fixed relation on the rotor core have previously been considered. One such technique involves positioning the magnetic elements around the core and applying a Kelvar or fiberglass wrap to hold the magnets in place. In this technique, an adhesive filler may be employed to fill voids and provide a more rigid structure, thereby further preventing magnet movement about the core during motor operation.
Another technique for retaining magnets on an associated core in a permanent magnet rotor includes wrapping a relatively fine wire, under tension, around the magnetic elements followed by an adhesive or epoxy coating to protect the wire and "pot" the assembly. Another technique employs a cylindrically shaped shell or "can" which is assembled around the outer peripheral surfaces of the magnets. Various prior methods and techniques for retaining magnets in fixed rotational position on associated cores in permanent magnet rotors are described in greater detail in the aforementioned U.S. Pat. No. 5,040,286.
While many of the prior approaches to retaining magnets on rotor cores has been found satisfactory for their intended applications, many exhibit drawbacks such as difficulty in manufacture and/or assembly, or failure to adequately retain the magnets in fixed rotational position on the rotors during high speed operation and when subjected repeated starting, stopping and reversal of the motor in which the rotor is used.
As described in pending application Ser. No. 863,900, public concern over power consumption, as well as environmental concerns, have among other things lead to increased research and effort in the design of more efficient electrical appliances, including components such as fan motors. Accordingly, a rotor assembly which lends itself to economical manufacturing techniques and use in motor applications which contribute to reduced power consumption and improved environmental conditions would greatly enhance the public well-being.